I believe I had a similar issue with getting some tests to run with Node + Mocha.

Are the tests that you are trying to run within the same Job as the Job that installs the node modules?

I was trying to separate my node setup(npm install) and the actual running of tests into two Jobs, which resulted in the node modules living in one build_dir and when the testing job was being ran, it was trying to do so within a different build_dir. Because the two jobs operated on two different build_dir's, when it came to running the tests, Bamboo could not find the appropriate modules.

My solution was to remove the second Job and put everything(separated by tasks) into one Job.